What are the odds there is life in outer space?

Ever since humans acknowledged the enormity of the universe, we have intuited that life must exist somewhere, either in our galaxy or some galaxy far, far away. If the­ universe contains billions of galaxies, and if each galaxy contains billions of stars, and if a fraction of those stars have Earth-like planets, then hundreds -- maybe even thousands -- of alien civilizations must exist across the cosmos. Right?

For a while, science contented itself with the logic alone. Then, in 1995, astronomers located the first planets outside our solar system. Since then, they've detected nearly 300 of these extra-solar planets. Although most are large, hot planets similar to Jupiter (which is why they're easier to find), smaller, Earth-like planets are beginning to reveal themselves. In June 2008, European astronomers found three planets, all a little larger than Earth, orbiting a star 42 light-years away [source: Vastag].

­These discoveries have served as an affirmation for those involved with the search for extraterrestrial intelligent life, or SETI. Harvard physicist and SETI leader Paul Horowitz boldly stated in a 1996 interview with TIME Magazine, "Intelligent life in the universe? Guaranteed. Intelligent life in our galaxy? So overwhelmingly likely that I'd give you almost any odds you'd like."

And yet his enthusiasm must be tempered by what scientists call the Fermi Paradox. This paradox, first articulated by nuclear physicist Enrico Fermi in 1950, asks the following questions: If extraterrestrials are so common, why haven't they visited? Why haven't they communicated with us? Or, finally, why haven't they left behind some residue of their existence, such as heat or light or some other electromagnetic offal?

Perhaps extraterrestrial life isn't so common after all. Or perhaps extraterrestrial life that gives rise to advanced civilizations isn't so common. If only astronomers could quantify those odds. If only they had a formula that accounted for all of the right variables related to extraterrestrial life. As it turns out, they do. In 1961, as a way to help convene the first serious conference on SETI, radio astronomer Frank Drake presented a formula, now known as the Drake Equation, that estimates the number of potential intelligent civilizations in our galaxy. The formula has generated much controversy, mainly because it leads to widely variable results. And yet it remains our one best way to quantify just how many extraterrestrials are out there trying to communicate.

Let's take a closer look at the equation and its implications.

Ellie Arroway, played by Jodie Foster in the movie "Contact," was consumed by the thought of life on other planets.

Are We Alone? The Drake Equation

Trying to calculate the probability that extraterrestrial life exists in the universe is actually quite complicated. The universe isn't a static environment. Stars are born, they live and they die. Some stars form in association with planets. Others don't. Only some of those planets have the right conditions to support life.

Life is a tricky variable in its own right. Some planets might support complex organic molecules -- proteins and nucleic acids -- and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. Finally, even organisms that have adapted extremely well to their environments don't last forever. As both the dinosaurs and the Roman Empire illustrate here on Earth, all dynasties come to an end, be it cataclysmic or otherwise.

Fra­nk D­rake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy -- and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:

N = RfpneflfifcL

In this equation, N is the number of detectable civilizations in our galaxy. The other variables are described below:

R is the rate of star formation in the galaxy

fp is the fraction of stars that form planets

ne is the number of planets hospitable to life (i.e., Earth-like planets)

fl is the fraction of these planets on which life actually emerges

fi is the fraction of these planets on which intelligent life arises

fc is the fraction of these planets with intelligent beings capable of interstellar communication

L is the length of time such a civilization remains detectable

The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year [source: Cain]. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.

Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets -- those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that ne equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:

N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000

which gives us a value of 12,000 civilizations in our galaxy.

Drake's original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way [source: Garber]. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy [source: Lemarchand]. That's a lot of ETs!

No wonder astronomers were so optimistic when they started searching diligently for extraterrestrial life in the 1960s. On the next page, we'll look at how they've conducted this search and what it has turned up.

Aerial view of the Arecibo Observatory in Puerto Rico

Photo courtesy of the NAIC - Arecibo Observatory, a facility of the NSF

Testing and Revising the Drake Equation

Armed with an estimate of the number of communicative civilizations in our galaxy, SETI scientists set out to find them. They had two basic options: face-to-face communication or long-distance communication. The former scenario required that extraterrestrials visit humans or vice versa. This seemed highly unlikely given the distances between our solar system and other stars in the Milky Way. The latter scenario involved radio broadcasts, either sending or receiving electromagnetic signals through space.

In 1974, astronomers intentionally transmitted a 210-byte message from the Arecibo Observatory in Puerto Rico in the hopes of signaling a civilization in the globular star cluster M13. The message contained fundamental information about humans and our corner of the universe, such as the atomic numbers of key elements and the chemical structure of DNA. But this sort of active communication has been rare. Astronomers mostly rely on passive communication -- listening for transmissions sent by alien civilizations.

A radio telescope is the tool of choice for such listening experiments because it's designed to detect longer-wavelength energy that optical telescopes can't see. In radio astronomy, a giant dish is pointed to a nearby, sunlike star and tuned to the microwave region of the electromagnetic spectrum. The microwave frequency band, between 1,000 megahertz and 3,000 megahertz (MHz), is ideal because it's less contaminated with unwanted noise. It also contains an emission line -- 1,420 MHz -- that astronomers can hear as a persistent hiss across the galaxy. This narrow line corresponds to energy transformations taking place in neutral hydrogen. As a primordial element of the universe, hydrogen should be known to all intergalactic civilizations, making it an ideal marker. Several teams from around the world have been systematically listening to stars across the Milky Way and adjacent galaxies since 1960.

Despite their collective efforts, no SETI search has received a confirmed, extraterrestrial signal. Our telescopes have picked up a few unexplained and intriguing signals, such as the so-called "Wow" signal detected by researchers at Ohio State University in 1977, but no transmission has been repeated in such a way that it provides indisputable evidence of extraterrestrial life. All of which brings us back to the Fermi Paradox: If thousands of civilizations in the Milky Way galaxy, why haven't we detected them?

Since Drake and Sagan made their estimates, astronomers have become more conservative. Paul Horowitz, who boldly guaranteed the existence of extraterrestrial life, has generated more modest results from the Drake Equation, finding that N may be closer to 1,000 civilizations [source: Crawford]. But even that figure may be too large.

In 2002, Skeptic magazine publisher Michael Shermer argued that astronomers weren't being critical enough in their evaluation of L, the length of time a civilization remains detectable. Looking at 60 civilizations that have existed on Earth since the dawn of humanity, Shermer came up with a value for L that ranged from 304.5 years to 420.6 years. If you plug these numbers into the Drake Equation, you find that N equals 2.44 and 3.36, respectively. Tweak the numbers some more, and you can easily get N to fall to one or even lower. Suddenly, the odds of hearing from an extraterrestrial life form are considerably lower.

Even the most enthusiastic SETI supporters are troubled by the lack of results produced by more than 40 years of "listening" to the cosmic airwaves. And yet most of that search has been confined to our home galaxy. Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. This tilts the odds again in favor of finding extraterrestrial life, which is why many SETI astronomers take the same approach to their work as lottery players: You can't win if you don't play.

What are the odds there is life in outer space?: Author's Note

As I worked on this piece, I couldn't stop thinking about Ellie Arroway, the heroine of Carl Sagan's book, "Contact" (and pictured on page two). There's a scene in the 1997 movie adapted from the book in which Arroway, lying atop her car in the New Mexico desert, hears the first tentative pulses of an alien civilization's greeting. She races back to the lab, shouting directions to her colleagues as she goes, trying to make sure the array of radio telescopes stays tuned to the signal. I still argue it's one of the most exciting scenes in modern American cinema. It made alien contact seem not just plausible, but imminent.

I knew nothing of the Drake Equation when I first watched "Contact." Then I received this assignment and came to grips with a harsh reality (at least if you're craning your neck, hoping to catch a glimpse of E.T.): Our galaxy may not be crowded with alien civilizations after all. Either the conditions don't exist to allow them to evolve, or if they do evolve, they kick off before we ever get a chance to meet them. All of which makes me wonder how Arroway would react to the Drake Equation. I have a feeling she would remain as optimistic as ever, clinging to her belief that the universe would be an "awful waste of space" if we were its sole inhabitants.

After years of searching and finding no ETs, many astronomers now think the values used in the Drake Equation should be ratcheted down. The implication: We may not be completely alone, but we're in no danger of having our personal space invaded.